Petroleum is currently estimated to account for over 35% of the world's total commercial primary energy consumption. Coal ranks second with 23% and natural gas third with 21%. The use of liquid hydrocarbon fuels on an enormous scale for transportation has led to the depletion of readily accessible petroleum reserves in politically stable regions and this, in turn, has focused attention, economically, technically and politically on the development of alternative sources of liquid transportation fuels. Liquid hydrocarbons are far and away the most convenient energy sources for transportation in view of their high volumetric energy. The energy density of gasoline, for example at about 9 kWh/liter and of road diesel at about 11 kWh/liter, far exceeds that of hydrogen (1.32 kWh/liter at 680 atm, or batteries, 175 Wh/kg. Furthermore, the liquid hydrocarbon fuel distribution infrastructure is efficient and already in place.
Conversion of coal into liquid hydrocarbon fuels has been used in the past in various countries to supplement or replace imported petroleum fuels, most notably by Germany during World War II when the Bergius and Fischer-Tropsch processes were used on a large scale and in the sporadically in the United States when petroleum crude prices were considered to have reached politically excessive levels. Processes for using large coal reserves have received attention in the United States, both in the direction of enabling coal to be burned with lower pollution emissions, e.g., Solvent Refined Coal, and in processes for converting coal into liquid fuels by alternative processes, for example, by the CO-Steam Process investigated at the Grand Forks Energy Research Center in North Dakota during a previous moment of national political panic. In this process, lignite is converted to boiler fuel by the reaction of a finely ground slurry of lignite in a hydrocarbon donor solvent with carbon monoxide, steam and hydrogen at about 450° C. and pressures up to about 35 MPa (5000 psi), as described in U.S. Pat. No. 4,337,142 (Knudson). Variants of this process subsequently considered but not brought to commercial use included the lower temperature aqueous-CO pretreatment process for low rank coals derived from terrestrial material, not significantly structurally different from lignin, peat and lignite coals. This process, described in U.S. Pat. No. 5,151,173 (Vaughan) significantly decreased the oxygen content of lignite and sub-bituminous (lower rank) coals, increased the H/C atomic ratio and increased the extractability of the coal without addition of molecular hydrogen. High pressures comparable to those used in the CO-Steam process were found to be necessary,
With the degree of attention being given currently, not so much to terrestrial petroleum shortage but, rather, to excess atmospheric greenhouse gases, the use of biomass as a source of liquid fuels is receiving widespread public and governmental attention and government subsidies for biomass research projects have become numerous. Biomass is considered to be desirable as a source of liquid fuels from biomass for the transportation sector because CO2 released from vehicle exhaust is captured during biomass growth making the process essentially carbon neutral. While direct, carbon-neutral use of biomass as fuel is established, for example, biodiesel, this route is limited because the limited choice of source materials, e.g., vegetable oils. Conversion of a wider variety of biomass sources into more traditional types of fuel, principally hydrocarbons, is the more attractive option.
Currently, there are two major routes for conversion of biomass to liquid fuels: biological and thermo-chemical. In the biological process, fermentation of easily fermentable plant products, such as, for example, sugars to alcohols is achieved. These easily fermentable plant products can be extracted from corn kernels, sugar cane and etc. The major disadvantage of this pathway is that only a fraction of the total carbon in biomass is converted to the final desired liquid fuel. It has been calculated that conversion of all corn produced in USA to ethanol can meet 12% of entire US demand for gasoline which reduces to 2.4% after accounting for fossil fuel input required to produce the ethanol. Similarly, an approximate estimate for the land area required to support the current oil consumption of about 2 million cubic meters per day by the US transportation sector is of the order of 2.67 million square km which represents 29% of the total US land area, using reasonable assumptions for the efficiency of the conversion process, thus suggesting that large scale production of liquid fuels from such a biomass conversion process is impractical.
While other processes for converting biomass to liquid fuels have been proposed, none has so far achieved large scale commercial success. Various problems exist, including major capital and operating expenses including high energy input requirements making the overall conversion unattractive and the need to use large process units to gain any reasonable production rate. Economics has therefore played a significant role in inhibiting the adoption of biomass conversion processes but since substitution of a part of the transportation fuel demand by biological materials would constitute a worthwhile economic, political and environmental advance, consideration is being given to various approaches.
Biomass oil provides one of the options which are being considered as a source of synthetic petroleum substitutes for fuel uses. It may be extracted by biomass-to-liquid technology involving destructive distillation of dried biomass in a reactor at temperature of about 500° C. with subsequent cooling. Biomass oil produced by rapid pyrolysis has been produced commercially on a small scale. Pyrolysis oil is a kind of tar and normally contains high levels of oxygen which preclude it from being considered as a direct hydrocarbon substitute. It is hydrocarbon insoluble, viscous, contains upwards of 20 wt % water along with 40-50 wt % organic oxygen compounds that decrease the heating value, and is unstable because sediment is formed via e.g., phenol-formaldehyde resin forming reactions that lead to coke formation on heating. Biomass oil produced by hydrothermal liquefaction is a higher grade hydrocarbon soluble oil with only about 15 wt % oxygen-containing organic compounds. Previous attempts to commercialize this approach have failed due to the high water usage and inability to feed the biomass effectively into the processing unit.